The following structures (substrates) each forming an acoustic surface wave device for processing different electric signals by utilizing acoustic surface waves, are already known.
(1) Structure consisting of only a substrate made of a piezoelectric substance (piezoelectric monocrystal substrate, piezoelectric ceramic substrate, etc.); PA1 (2) Structure consisting of a non piezoelectric substrate and a piezoelectric layer formed thereon; and PA1 (3) Structure consisting of a semiconductor substrate and a piezoelectric layer formed thereon. PA1 (1) Since a high quality layer is difficult to obtain, it is not possible to form a layer with a satisfactorily high reproducibility with respect to its piezoelectricity. PA1 (2) Propagation loss of acoustic surface waves is high in the high frequency regions. PA1 (3) Fluctuation in propagation characteristics of the acoustic surface waves is high. PA1 (4) It is difficult to control the variation ratio in delay time (.tau.) in dependence on the temperature for acoustic surface waves (1/.tau.).multidot.(.differential..tau./.differential.T) (T: room temperature).
Among multilayer structures of the type mentioned in item 2, at present a structure consisting of a substrate made of sapphire or glass and a layer made of zinc oxide (ZnO) formed thereon by sputtering or another method is most widely utilized. However, this ZnO layer has the following disadvantages and therefore it is difficult to use in practice.